Planar optical waveguides are required for optical interconnection in integrated optical and optoelectronic devices. The methods used to manufacture such waveguides must be compatible with semiconductor processing methods used to manufacture other parts of the integrated devices.
Planar optical waveguides have been made by depositing a photosensitive monomer on a substrate and selectively exposing the deposited monomer to ultraviolet (UV) radiation. The UV radiation polymerizes the exposed monomer to provide polymer regions having a relatively high refractive index bounded by monomer regions having a relatively low refractive index. A further monomer layer is generally deposited over the partially polymerized layer for protection against surface flaws and contaminants which could couple light out of the polymerized regions. Unfortunately, the waveguides made by this method are unstable at the high temperatures which are used in some semiconductor processing methods. Consequently, all high temperature processing steps must be completed before the waveguides are defined. Moreover, this method generally requires two or more deposition steps.
Planar optical waveguides have also been made by depositing or growing a first layer of Si0.sub.2 on a substrate, depositing a layer of Si.sub.3 N.sub.4 on the first layer of Si0.sub.2, depositing a second layer of Si0.sub.2 on the Si.sub.3 N.sub.4 layer, and selectively removing a partial thickness of the second Si0.sub.2 layer in selected regions to lower the effective refractive index of the underlying Si.sub.3 N.sub.4 layer in those regions. This method requires three deposition or growth steps and one etch back step, all of which must be carefully controlled for satisfactory results.
Silicon-based planar optical waveguides have also been made by depositing or growing a first layer of undoped Si0.sub.2 on a substrate, depositing P-doped Si0.sub.2 on the layer of undoped Si0.sub.2, selectively removing regions of the P-doped Si0.sub.2 layer to expose regions of the first layer of undoped Si0.sub.2, and depositing a second layer of undoped Si0.sub.2 on the exposed regions of the first layer of undoped Si0.sub.2 and on the remaining regions of P-doped Si0.sub.2. The regions of P-doped Si0.sub.2 have a higher refractive index than the surrounding regions of undoped Si0.sub.2. This method also requires three deposition or growth steps and one etch back step, all of which must be carefully controlled for satisfactory results.
In U.S. Pat. No. 4,585,299, Robert J. Strain discloses a method for making silica-based planar optical waveguides in which boron, phosphorus, arsenic or germanium is implanted into a silicon substrate through a first mask and the substrate is oxidized through a second mask to provide a patterned Si0.sub.2 layer which incorporates the implanted dopant. The implanted dopant raises the refractive index of a central region of the Si0.sub.2 layer to provide a waveguide. This patent suggests that migration of the dopant during the oxide growth may be a problem.
Silicon-based planar optical waveguides have also been made by depositing or growing a layer of Si0.sub.2 on a substrate and selectively bombarding the Si0.sub.2 with H or B ions to define regions having a relatively high refractive index bounded by regions having a relatively low refractive index. The implantation process causes localized compaction of the Si0.sub.2 which locally increases the refractive index of the Si0.sub.2. The presence of the implanted H or B ions may also modify the refractive index of the implanted Si0.sub.2. Unfortunately, the Si0.sub.2 is decompacted and the implanted H or B ions are redistributed by diffusion in the Si0.sub.2 layer if the waveguides are subjected to subsequent high temperature processing steps. The decompaction of the Si0.sub.2 and the migration of the implanted H or B ions degrades the refractive index profile defined by the implantation process. Consequently, all high temperature processing steps must be completed before the waveguides are defined.